Cancer metastasis accounts for the vast majority of cancer-related fatalities and remains a challenge for both clinicians and scientists. The epithelia mesenchymal transition (EMT) contributes to cancer metastasis by activating genes required for cell migration and invasion. Snail1 is a key regulator of EMT, and Snail1's protein level, subcellular localization, and function are tightly regulated in epithelial cells. We developed a high throughput screen with a human kinome RNAi library to identify novel post-translational regulators of Snail1. The discoidin domain receptor 2 (DDR2) was identified through this screen and we found that DDR2 depletion resulted in decreased levels of Snail1, while DDR2 overexpression correlated with elevated levels of Snail1. We have shown that DDR2 stabilized Snail1 protein post-translationally with little change in Snail1 mRNA levels, and this stabilizatio was dependent on MAPK/ERK signaling through ERK2. Additionally, DDR2 was expressed in human invasive ductal breast cancer, but not normal breast tissue or invasive lobular breast cancer. These preliminary findings indicate that DDR2 likely plays an important role in regulating Snail1-mediated cell invasion during breast cancer metastasis. However, the molecular mechanism by which DDR2 signals to stabilize Snail1 protein and whether DDR2 is essential for Snail1- mediated breast cancer cell invasion in vivo is unknown and will be studied through three specific aims in this proposal. The role of ERK2 in Snail1 stabilization will be studied by determining how ERK2 and Snail1 physically interact, whether ERK2 phosphorylates Snail1 and at which sites, and how this affects the cellular function of Snail1. In addition, the in vivo roleof DDR2 in breast cancer cell invasion will be determined using two different model systems: the chicken chorioallantoic membrane (CAM) assay and a human-in-mouse intraductal (MIND) xenograft model. Finally, DDR2 expression will be measured in multiple subtypes of human breast cancer, and whether this expression correlates with Snail1 expression or other breast cancer prognostic factors will be determined. This research will unravel novel molecular mechanisms regulating EMT in cancer that contribute to invasion and metastasis, and will help to develop better therapies for treating breast cancer patients in the future.